The present invention relates generally to biologically active compounds and more specifically to compounds and peptides which are amphipathic, i.e., have both hydrophilic and hydrophobic portions. Specifically, the invention relates to improved methods for the delivery and presentation of amphipathic peptides in association with micelles diagnostic, therapeutic, cosmetic and organ, tissue and cell preservative uses.
Of particular interest to the present invention are the biologically active amphipathic peptides which are members of the family of peptide compounds including vasoactive intestinal peptide (VIP), growth hormone releasing factor (GRF), peptide histidine isoleucine (PHI), peptide histidine methionine (PHM), pituitary adenylate cyclase activating peptide (PACAP), gastric inhibitory hormone (GIP), hemodermin, the growth hormone releasing hormone (GHRH), sauvagine and utotensin I, secretin and glucagon. More specifically, the invention relates to improved therapeutic methods for delivering peptides in the VIP/GRF family of peptides to targeted tissues through use of improved micelle compositions comprising a member of the VIP/GRF family of peptides and biologically active analogues thereof.
VIP is a 28-amino acid neuropeptide which is known to display a broad profile of biological actions and to activate multiple signal transducing pathways. See, Said, Peptides 5 (Suppl. 1):149-150 (1984) and Paul and Ebadi, Neurochem. Int. 23:197-214 (1993). A Schiff-Edmundson projection of VIP as a .pi.-helix reveals segregation of apolar and polar residues onto the opposite faces of the helix and that this amphipathic character is also evident when VIP is modeled as a distorted .alpha.-helix, which is reported in Musso, et al., Biochemistry 27:8147-8181 (1988). A correlation between the helix-forming tendency of VIP analogues and their biological activity is described in Bodanszky et al., Bioorgan. Chem. 3:133-140 (1974). In pure water, the spectral characteristics of VIP are consistent with those of a random coil. However, organic solvents and anionic lipids induce helical-information in the molecule. See, Robinson et al., Biopolymers 21:1217-1228 (1983); Hamed, et al., Biopolymers 22:1003-1021 (1983); and Bodanszky, et al., Bioorganic Chem. 3:133-140 (1974).
Short peptides capable of forming amphipathic helices are known to bind and penetrate lipid bilayers. See, Kaiser and Kezdy, Ann. Rev. Biophys. Biophysical Chem. 15:561-581 (1987) and Sansom, Prog. Biophys. Molec. Biol. 55:139-235 (1991). Examples include model peptides like (LKKLLKL-), which are disclosed in DeGrado and Lear, J. Am. Chem. Soc. 107:7684-7689 (1985), and the 26-residue bee venom peptide, melittin, disclosed in Watata and Gwozdzinski, Chem-Biol. Interactions 82:135-149 (1992). Possible mechanisms for the binding include alignment of peptide monomers parallel to the surface of the bilayer mediated by electrostatic interactions between polar amino, acids and phospholipid head groups, and insertion of peptide aggregates into the apolar bilayer core, stabilized in part, by the hydrophobic effect. See, Sansom, Prog. Biophys. Molec. Biol. 55:139-235 (1991).
VIP belongs to a family of homologous peptides, other members of which include peptide histidine isoleucine (PHI), peptide histidine methionine (PHM), growth hormone releasing factor (GRF), pituitary adenylate cyclase activating peptide (PACAP), gastric inhibitory hormone (GIP), hemodermin, the growth hormone releasing hormone (GHRH), sauvagine and utotensin I, secretin and glucagon. Like VIP, the other members of the VIP/GRF family of peptides, and biologically active analogues thereof, can form amphipathic helices capable of binding lipid bilayers. The biological action of members of the VIP/GRF family of peptides are believed to be mediated by protein receptors expressed on the cell surface and intracellular receptors and it has recently been demonstrated that calmodulin is likely to be the intracellular receptor for VIP [Staliwood, et al., J. Bio. Chem. 267:19617-19621 (1992); and Stallwood, et al., FASEB J. 7:1054 (1993)].
Bodanszky et al., Bioorgan. Chem. 3:133-140 (1974) were the first to study the conformation of VIP through optical rotary dispersion and circular dichroism spectrum. They showed structural differences in VIP, depending on the hydrophobicity of the solvent in which VIP was dissolved. The VIP-in-water spectrum revealed a mostly random coil structure. (about 80%). However, addition of organic solvents, such as tifluoroethanol (TFE) or methanol, even at low concentration induced a pronounced shift to a helical structure. The authors suggested that these effects of the organic solvents on the structure of the peptide would coincide with receptor conditions, and therefore, the helical conformation of VIP would correspond to an "active architecture" required for its biological activity. These early studies were in agreement with the findings of Robinson et al., Biopolymers 21:1217-1228 (1982), who analyzed the conformation of VIP, and two of its family members, secretin and glucagon, in water, anionic detergents, and anionic lipids (PA and phosphatidylglycerol (PG)). They showed an increase in the helix formation probability by arginyl, histidyl, and lysil residues, corresponding in all three peptides to their 13-20 amino acid region. A predominantly disordered structure was again observed for VIP in aqueous solvents, and zwitterionic lipids, suggesting that the charge of the polar head group plays an important role in helix formation. Using circular dichroism (CD) spectra studies with 40% HFIP/H.sub.2 O mixture and .sup.1 H-NMR studies Fournier et al., Peptides 5:160-177 (1984), showed that the 15-28 portion of the VIP segment forms an a-helix in the presence of organic solvent. A complete structural study of the native VIP in 40% TFE was performed by Theriault et al., Biopolymers 31:459-464 (1991) using two-dimensional .sup.1 H-NMR spectroscopy. Their results were similar to the ones obtained by Fry et al., Biochemistry 28:2399-2409 (1989) who investigated VIP in 25% methanol/water. They described two helical segments between the amino acids 7-15 and 19-27 linked by a random coil peptide chain portion that granted mobility to the molecule.
Finally several groups worked on the development of more potent analogs of VIP as potential therapeutic agents, since the native peptide had a very low bioavailability. Interestingly, all of them modified the sequence of VIP to enhance its helicity and amphiphilicity. VIP structure-activity relationship were studied extensively by Bolin and his collaborators (Fry et al., Biochemistry 28:2399-2409 (1989); Bolin et al., Biopolymers 37:57-66 (1995). Among their results, the enhancement of the helical structure by specific substitutions of amino acid residues was proportionally related to an increase in potency, and the pharmacoactive functional group of the VIP was found to consist of multiple binding sites throughout the entire peptide sequence. Helix based analogs of VIP were also developed by Musso et al., Biochemistry 27:8174-8181 (1988) that showed greater interactions with receptors. Stearyl-Norleucine-VIP analog that has a 100-fold greater potency was designed by Gozes et al., Endocrinology 134:2121-2125 (1994), for noninvasive impotence treatment and neurodegenerative diseases Gozes et al. J. Pharmacol. Exp. Ther. 273:161-167 (1996). The addition of fatty acid moiety and the amino acid substitutions increased the lipophilicity of the peptide, which was believed to improve biological membrane penetration.
In summary, VIP has been shown to adopt a helical conformation in hydrophobic environments, provided by organic solvent, and the helical structure of the VIP increases with an increase in the hydrophobicity of the environment. This helical motif found in the central part of the peptide, which is rich in basic, hydrophobic residues, forms an amphiphilic structure that may facilitate the binding to receptors and promote direct interactions with membrane lipids, causing an increase in bioactivity. Furthermore, it is possible that the helical structure of VIP also contributes to an increased stability, by protecting specific sites particularly sensitive to proteolytic degradation.
As reviewed by Gozes et al., Mol. Neurobiol. 3:201-236 (1989), immunofluorescence and radioimmunoassay techniques demonstrated the wide but selective distribution of VIP in the central and peripheral nervous systems. In the brain, the highest density of VIP-rich neurons occur in the hypothalamus, particularly in the suprachiasmatic and paraventricular nuclei and in the cerebral cortex. VIP concentrations are higher in the hyposphyseaal portal blood than in the peripheral blood, indicating secretion of the peptide by the hypothalamus and its transport to the adenohypophysis. In the peripheral nervous system, VIP-immunoreactive nerves are found in fibers and terminals that supply blood vessels, nonvascular smooth muscle, and glandular acini and ducts in many organs. Coexistence of VIP with acetylcholine in cholinergic neurons is also well-documented. Some VIP nerves have recently been acknowledged to be components of the autonomic nervous system. Furthermore, Muller et al., Mol. Neurobiol. 10:115-134 (1995) showed that a distinct groups of cells, such as platelets, mast cells, skin cells, neutrophils, and retinal amacrine cells appear to be able to synthesize and release VIP.
The physiologic effects of VIP are largely mediated by its binding to specific cell receptors. Hirata et al., Biochem. Biophys. Res. Comm. 132:1079-1087 (1985) described two specific receptor binding sites for VIP, one low-, on high-affinity, on cultured vascular smooth: muscle cells from rat aorta, that were distinct from .beta.-adrenergic receptors. From a molecular aspect two distinct polyvalent VIP receptors were distinguished after cloning of cDNAs. The first, VIP.sub.1, receptor is similar to the secretin receptor also called PACAP type II receptor, is expressed in intestine, lung, liver, muscle cells, ovaries, and various brain regions (Sreedharan et al., Biochem. Biophys. Res. Comm. 203:141-148 (1994)). The second, VIP.sub.2 receptor is closer to the GRF binding site and has a distinct distribution in the central nervous system (Lutz et al., FEBS Let. 334:3-8 (1993)). Recent studies from our lab also indicated that VIP action can be non-receptor mediated (Sejourne et al., Pharm. Res. 14(3):362-365 (1997)).
Although studied for many years, most of the intracellular signaling cascades of VIP remain to be elucidated. Most common cellular action observed in many cells is the increased production of intracellular cyclic adenosine monophosphate (cAMP), via the stimulation of adenylate cyclase. The subsequent steps of cAMP-induced pathways are still highly speculative. Conversely, several observations indicate the existence of cAMP-independent signal transduction cascades. Sreedharan et al., Biochem. Biophys. Res. Comm. 203:141-148 (1994) recently found that VIP.sub.1 receptor induced two separate pathways in one cell type, i.e. activation of adenylate cyclase and increase in intracellular Ca.sup.2+. Stimulation of adrenal medulla and cervical ganglion by VIP were shown to increase the generation of inositol 1,4,5 triphosphate (IP.sub.3) and intracellular Ca.sup.2+ (Malhotra et al., J. Biol. Chem. 263:2123-2126 (1988)). Moreover, it has been proposed that internalized VIP could bind to nuclear receptors and activate protein kinase C (Omary et al., Science 238:1578-1580 (1987); Zom et al., Biochem. Pharmacol. 40:2689-2694 (1990)).
The pleiotorpic distribution of VIP is correlated with its involvement in a broad spectrum of biological activities, and growing evidence suggests that VIP plays a major role in regulating a variety of important functions in many organs. Physiological actions of VIP have been reported on the cardiovascular, respiratory, reproductive, digestive, immune, and central nervous systems, as well as metabolic, endocrine aid neuroendocrine functions (for review, Said, Trends Endocinol. Metab. 2:107-112 (1991)). In many cases, VIP acts as a neurotransmitter or neuromodulator and released into the local circulation at small concentrations. Among the functions that VIP is believed to mediate or promote, are (Said, Trends Endocrinol. Metab. 2:107-112 (1991) Paul et al., Neurochem. Int. 23:197-214 (1993)) the vasodilation of cerebral, coronary, peripheral, and pulmonary blood vessels, linked to the regulation of vascular tone; the relaxation of gastrointestinal, uterine, and tracheobroncial smooth muscles; exocrine secretion, water and anions by intestinal, respiratory, and pancreatic epithelia; stimulation of the male and female activity and responses; release and regulation of neuroendocrine functions (renin release, melatonin secretion); inhibition of the immune system (inhibition of platelet aggregation); and stimulation and protection of neuronal cells.
New VIP functions such as inhibition of vascular smooth muscle cell growth, proliferation of cultured human keratinocytes, the release of neutrophic and growth factors involved in cell differentiation and ontogeny, and antioxidant properties have been recently proposed but still need additional studies (Muller et al., Mol. Neurobiol. 10:115-134 (1995); Said, Trends Endocrinol. Metab. 2:107-112 (1991)).
Some human diseases today are known to be associated with the deficiency in the release of VIP. The deficiency of VIP has been linked to the pathogenesis of several diseases, such as cystic fibrosis, diabetic impotence, congenital mengacolon in Hirschsprung's disease, and achalasia of the esophagus. Furthermore, VIP insufficiency may be a cause of bronchial hyperactivity in asthmatic airways since VIP is known to mediate airway relaxation in humans, and lung tissues of asthmatic patients showed a selective absence of VIP nerves (Ollerenshaw et al., N. Engl. J. Med. 320:1244-1248 (1989)). Finally, Avidor et al., Brain Res. 503:304-307 (1989) observed an increase in brain VIP gene expression in a rat model for spontaneous hypertension, thought to be associated with the pathophysiology of the disease.
On the other hand, the excessive release of VIP has been linked to the pathogenesis of few diseases. One of the pathological syndromes is pancreatic cholera, a watery diarrhea-hypocholaremia-hypochloridria condition (Krejs, Ann. N.Y. Acad. Sci. 527:501-507 (1988)). Certain tumors, especially pancreatic, bronchogenic, and neurogenic, have been associated with elevated circulatory levels of VIP.
Due to the numerous physiological actions of VIP, the use of VIP as a drug has been of growing interest. The potential therapeutic developments of VIP include treatment of diseases where regional blood flow is deprived. These include hypertension by reducing systemic vascular overload, left ventricular failure, congestive heart failure, and coronary or peripheral ischemia. VIP infusion in man for 10 hours was shown to reduce total peripheral resistance by 30% and increase forearm blood flow by 270% (Frase et al., Am. J. Cardiol. 60:1356-1361 (1987)). Moreover, Smiley, Am. J. Med. Sci. 304:319-333 (1992) showed VIP-immunoreactive nerves in the skin and plasma levels of VIP were found to be low in patients with schleroderma, thus treatment with VIP may restore this impaired response. Other diseases which could be treated by administration of VIP include treatment of asthmatic bronchospasm. VIP has been shown to protect against bronchoconstriction in asthmatic patients and as a relaxant of tracheobronchial smooth muscle (Morice et al., Lancet 26 2(8361):1225-1227 (1983)). Its anti-inflammatory properties could further enhance its therapeutic value in asthma (Said, Biomed. Res. 13 (Suppl. 2):257-262 (1992)). Administration of VIP could also be used in the prevention and/or reduction of tissue injury. The peptide has been described to prevent neuronal cell death produced by the external envelope protein gp 120 of the human immunodeficiency virus in vitro (Gozes et al., Mol. Neurobiol. 3:201-236 (1989); Hokfelt, Neuron. 7:867-879 (1991)), which may lead to a potential therapy for AIDS dementia as well as treatment of Alzheimer's disease. Likewise, the acute inflammatory lung injury induced by a variety of insults including oxidant stress was diminished by the presence of VIP (Berisha et al., Am. J. Physiol. 259:L151-L155 (1990)). VIP added to certain pneumoplegic solutions was also showed to improve rat lung preservation before transplantation (Alessandrini et al., Transplantation 56:964-973 (1993)).
A major factor limiting in vivo administration of VIP has been its reduced bioavailability at target tissues mostly because of proteolytic degradation, hydrolysis, and/or a multiplicity of conformations adopted by the peptide. It has been speculated that intracellular delivery of VIP alone and/or VIP-calmodulin mixtures could bypass the requirement for cell-surface binding of the peptide and thus enhance the biological actions of the peptide. Provision of the peptides expressed in and on liposomes would possibly permit intracellular delivery, since lipid bilayers of liposomes are known to fuse with the plasma membrane of cells and deliver entrapped contents into the intracellular compartment.
Liposomes are microscopic spherical structures composed of phospholipids which were first discovered in the early 1960s (Bangham et al., J. Mol. Biol. 13:238 (1965)). In aqueous media, phospholipid molecules being amphiphilic spontaneously organize themselves in self-closed bilayers as a result of hydrophilic and hydrophobic interactions. The resulting vesicles, called liposomes, therefore encapsulate in their interior part of the aqueous medium in which they are suspended, property that makes them potential carriers for biologically active hydrophilic molecules and drugs in vivo. Lipophilic agents could also be transported, embedded in the liposomal membrane. However, the success of liposomes in medical applications has been severely limited by their rapid sequestration in the reticuloendothelial system (RES). Efforts to reduce the RES uptake of liposomes led in the late 1980s to the development of liposomes with a significant increase in their circulation half-lives (sterically stabilized liposomes) (SSL), and revived hopes for their development as drug delivery systems. Two independent laboratories, from studying the biology of red blood cells, identified the presence of sialic acid on the membrane of erythrocytes to be partly responsible for their very long circulation times. Indeed, the incorporation of sialated glycolipids such as the ganglioside GM, into phosphatidylcholine (PC):cholesterol (Chol) liposomes effectively increased the circulation time of the vesicles (Allen et al., FEBS Letter 223:42-46 (1987); Allen et al., U.S. Pat. No. 4,920,016, Appl. 132,136, Dec 18, 1987; 24 pp, Apr 24, 1990; Gabizon et al., Proc. Natl. Acad. Sci. USA 8:6949 (1988)). These first results have raised new perspectives for liposomes as drug carriers, especially in the field of chemotherapy, since longer half-lives correlated well with higher uptake by implanted tumors in mice (Gabizon et al., Proc. Natl. Acad. Sci. USA 8:6949 (1988)).
In the 1990s, the near simultaneous development by several investigators of the second generation of SSL containing lipid derivatives of polyethylene glycol (PEG) resulted in further improvements (Klibanov et al., FEBS Letter 268 (1):235-237 (1990); Allen et al., Biochim. Biophys. Acta 1066:29-36 (1991)). Klibanov et al., FEBS Letter 268 (1):235-237 (1990) demonstrated that the blood clearance half-life of PC/Chol (1:1) liposomes in mice was 30 min vs. 5 hours for vesicles composed of PC/Chol/PEG-PE (1:1:0.15). Besides, the preparation techniques of the conjugated phospholipid PEG-di-steroyl-phosphatidylethanolamine (DSPE) were reported to be quick and simple (Klibanov et al., FEBS Letter 268 (1):235-237 (1990); Allen et al., Biochim. Biophys. Acta 1066:29-36 (1991), and PEG had already received approval for pharmaceutical use (PEG-ADA, Rhinaris.RTM.).
Of interest to the present invention is the observation of increased half-life of circulating protein through conjugation of the protein to a water soluble polymer [Nucci, et al., Adv. Drug Del. Rev. 6:133-151 (1991); Woodle, et al., Proc. Intern. Symp. Contro. Rel. Bioact. Mater. 17:77-78 (1990)]. This observation led to the development of sterically stabilized liposomes (SSL) (also known as "PEG-liposomes") as an improved drug delivery system which has significantly minimized the occurrence of rapid clearance of liposomes from circulation. [Lasic and Martin, Stealth Liposomes, CRC Press, Inc., Boca Raton, Fla. (1995)]. SSL are polymer-coated liposomes, wherein the polymer, preferably polyethylene glycol (PEG), is covalently conjugated to one of the phospholipids and provides a hydrophilic cloud outside the vesicle bilayer. This steric barrier delays the recognition by opsonins, allowing SSL to remain in circulation much longer than conventional liposomes [Lasic and Martin, Stealth Liposomes, CRC Press, Inc., Boca Raton, Fla. (1995); Woodle, et al., Biochem. Biophys. Acta 1105:193-200 (1992); Litzinger, et al., Biochem. Biophys. Acta 1190:99-107 (1994); Bedu Addo, et al., Pharm. Res. 13:718-724 (1996)] and increases the pharmacological efficacy of encapsulated agents, as demonstrated for some chemotherapeutic and anti-infectious drugs [Lasic and Martin, Stealth Liposomes, CRC Press, Inc., Boca Raton, Fla. (1995)]. Studies in this area have demonstrated that different factors affect circulation half-life of SSL, and ideally, the mean vesicle diameter should be under 200 nm, with PEG at a molecular weight of approximately 2,000 Da at a concentration of 5% (9-12) [Lasic and Martin, Stealth Liposomes, CRC Press, Inc., Boca Raton, Fla. (1995); Woodle, et al., Biochem. Biophys. Acta 1105:193-200 (1992); Litzinger, et al., Biochem. Biophys. Acta 1190:99-107 (1994); Bedu Addo, et al., Pharm. Res. 13:718-724 (1996)].
The mechanism by which SSL avoids macrophages and circulate longer in the blood is thought to involve the formation of a "steric barrier" around the liposomes by the attached PEG molecules. Torchilin, et al., Stealth Liposomes, D. Lasic and F. Martin (Eds.), CRC Press, Boca Raton, Fla., pp. 51-62 (1995) claimed that the ability of PEG to prevent liposome opsonization is determined by its behavior in the solvent which entails the formation of a hydrophilic cloud over the vesicle surface even at relatively low polymer concentrations. This negative, hydrophilic coat would act as a protective shield delaying the binding of opsonins that are often attracted to the positive charged lipid surfaces.
The circulation time of sterically stabilied liposomes may be controlled by selection of their size, PEG molecular weight, chain length and concentration and selection of the lipid composition. Maruyama et al., Chem. Pharm. Bull. 39:1620-1622 (1991) tested SSL with different PEG molecular weights (1,000, 2,000, 5,000, and 12,000 Da), with a constant size (180 to 200 nm) and composition (6% DSPE-PEG in DSPC/Chol (1:1)). The PEG.sub.2,000 -liposomes appeared to be the longest lasting formulation in mice, with 47.1% of injected dose after 6 h still in the blood. Klibanov et al., FEBS Letter 268 (1):235-237 (1990) conducted similar studies on mice with PC/Chol/PEG-PE (10:5:1) extruded liposomes of 200 nm diameters, using PEG.sub.750, PEG.sub.2,000 and PEG.sub.5,000. The authors evaluated the "degree of steric barrier" produced on the liposome surface and concluded that it was directly correlated to chain length of PEG and concentration-dependent. They suggested that the SSL prolongation was directly proportional to PEG chain length, which, itself, corresponded to the steric barrier. Finally, other groups (Allen et al., Biochim. Biophys. Acta 1066:29-36 (1991); Woodle et al., Biochim. Biophys. Acta 1105:193-200 (1992)) found somewhat contradictory results showing that the extension of PEG chain length from 2,000 to 5,000 Da had no additional suppression effect on RES uptake. PEG of molecular weights 1,900, 2,000 and 5,000 have been recently used in various applications.
Huang's group (Klibanov et al., Biochim. Biophys. Acta 1062:142-148 (1991); Litzinger et al., Biochim. Biophys. Acta 1190:99-107 (1994) pointed out the importance of the size of liposomes in biodistribution studies, and observed that small vesicles (&lt;100 nm) were taken up by the liver, whereas larger ones (300 nm&lt;diameter&lt;500 nm) were also accumulated in the spleen, particularly in the red pulp and marginal zone. Indeed, the major function of the spleen is to filter the aged or damaged red blood cells, and the liposome uptake was shown to use this same filtration mechanism, followed by splenic macrophage endocytosis. The reason for such an uptake is however unknown. Their studies showed an optimi circulation time for SSL of 150-200 nm diameters. Ghosh et al., Stealth Liposomes, D. Lasic and F. Martin (Eds.), CRC Press, Boca Raton, Fla., pp. 13-24 (1995) confirmed this work, showing the limitation of the prolongation effect of SSL to a narrow size range, between 70 and 200 nm diameter. Most of SSL applications seem indeed to include a size reduction step in their liposome preparation methods.
Klibanov et al., Biochim. Biophys. Acta 1062:142-148 (1991) studied the effect of the lipid composition on the blood circulation time of SSL, and found that the half-lives of different SSL were all very close, except when phosphatidylserine (PS) was added. Woodle et al., Biochim. Biophys. Acta 1105:193-200 (1992) also conducted biodistribution studies on mice and rats with SSL of various lipid compositions. They showed similarly that an increase in the hydrogenation of PC (i.e. bulk lipid transition temperature), the addition of the anionic lipid PG, and different levels of cholesterol had no impact on the prolongation effect. A consistent half-life of about 15 h for blood clearance was observed, regardless of the phospholipids phase transition, cholesterol content or neutral/negative charges.
Nevertheless, Bedu-Addoetal., Pharm. Res. 13:718-724(1996) recently shed light on the role of cholesterol in the stabilization of liposomes, claiming that the most suitable formulation for prolonged circulation times should contain a minimum of 30 mol % cholesterol, with low concentrations of short-chain PEG-PE (&lt;10%). The authors investigated the efficiency of surface protection in vitro using a fluorescence energy transfer technique. The addition of cholesterol improved surface protection, due to the increase in bilayer cohesive strength. It would limit the formation of "bald spots" less enriched with PEG-PE in the liposomal bilayer, thus inhibiting phase separation and lipid exchange with blood lipoproteins. However, in vivo, it was shown that the long-lasting circulation of SSL seems to depend mostly on the PEG coating and less on the liposome bilayer composition.
Different investigators reported that only 5% PEG-PE could give an optimized steric barrier effect on the vesicles (Klibanov et al., Biochim. Biophys. Acta 1062:142-148 (1991); Woodle et al., Biochim. Biophys. Acta 1105:193-200 (1992); McIntosh et al., Stealth Liposomes, D. Lasic and F. Martin (Eds.), CRC Press, Boca Raton, Fla., pp. 63-71 (1995)). A maximal limit of 10 mol % PEG was very recently proposed to obtain adequate results from in vitro studies, because of the spontaneous formation of micelles of PEG-PE at higher concentrations (Bedu-Addo et al., Pharm. Res. 13:718-724 (1996)).
Also of interest to the present application is the disclosure of PCT Application PCT/US97/05161 relating to improvements in sterically stabilized liposomes and therapeutic and diagnostic including acoustic diagnostic methods of using same.
Of interest to the present invention is work relating to molecular aggregates called "micelles" which are defined as colloidal aggregates spontaneously formed by amphiphilic compounds in water above a critical solute concentration, the critical micellar concentration (CMC), and at solution temperatures above the critical micellar temperature (CMT). The molecules constituting the micelles are in rapid dynamic equilibrium with the unassociated molecules. The increase in the concentration above the CMC usually leads to an increase in the number of micelles without any change in micellar size; however, in certain cases with phospholipid mixed micelles, the spherical micelles enlarge into rod-shaped micelles (Carey et al., Arch. Inter Med. 130:506-527 (1972); Hjelm, Jr. et al., J. Phys. Chem. 96 (21):8653-8661 (1992)). The CMC is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range (Almgren et al., Colloid Polym. Sci. 273:2-15 (1995)). An increase in the temperature leads to an increase in the number of aggregates, while the hydrodynamic radius remains constant (Nivaggioli et al., Langmuir. 11 (3):730-737 (1995); Alexandridis et al., Langmuir. 11:1468-1476 (1995)). In general the increase in temperature leads to an increase in hydrophobic interactions and the water dielectric constant is reduced augmenting the ionic repulsion forces. There are many ways to determine the CMC of an amphiphilic compound (surface tension measurements, solubilization of water insoluble dye, conductivity measurements, light scattering, and the like). According to a preferred method, surface tension measurements may be used to determine the CMC of PEG-DSPE micelles at room temperature.
Surfactant micelles are used as adjuvants and drug carrier systems in many areas of pharmaceutical technology. Micelles have been used to increase bioavailability or decrease adverse effects of the drugs (Trubetskoy et al., Advan. Drug Deliv. Reviews 16:311-320 (1995). In addition, the small size of micelles play a key role in transport across membranes including the blood brain barrier (Muranushi et al., Chemistry and Physics of Lipids 28:269-279 (1981); Saletu et al., Int. Clin. Psychopharmacol. 3:287-323 (1988). The surfactant micelles are thermodynamically unstable in aqueous media and subject to dissociation upon dilution. Yokoyania et al., Makromol Chem. Rapid Commun. 8:431-435 (1987) proposed a class of amphiphilic polymers, such as polyethylene glycol (PEG), which are known to form more stable polymeric micelles in aqueous solutions. There are many advantages to polymeric micelles, such as small size might control penetration across physiological barriers, increases the half-life in vivo, and allows to target micelles to specific tissues.
Studies involving polymer conjugated lipid micelles, such as PEG conjugated to PE are very recent. In fact only one such study has been done so far to our knowledge, where polyethylene-oxide (PEO) is conjugated to PE and dissolved in aqueous media forming micelles. The study performed by Triubetskoy et al., Acad. Radiol. 3:232-238 (1996) used PEO-PE conjugated lipid to encapsulate indium-111 and gadolinium chalets as contrast media for precutaneous lymphography using magnetic resonance imaging (MRI) topography. The study concluded that PEO-PE micelles can incorporate amphiphilic agents and prolong their actions in vivo by avoiding the RES, and prolonging the circulation period.
The stability of amphiphilic micelles depends on the strength of Van der Waals interactions. The polymer presence on the micellar surface contributes to its steric protection by repulsive action of the hydrophilic layer from the hydrophobicity of macrophages, thus decreasing the uptake by reticuloendothelial system (RES). Furthermore, the negative charge of the polymer creates a repulsive steric effect in vivo that prevents the binding of opsonins, plasma protein that facilitates RES uptake (Trubetskoy et al., Proceed. Intern. Symp. Control. Tel. Bioact. Mater. 22:452-453 (1995)). Thus, the polar and electrostatic interactions of the polymer with the in vivo environment is responsible for the steric stabilization of phospholipid micelles in vivo.
For sterically stabilized phospholipid micelles (SSM) formation an optimal amphiphilic compound is required, one with the right amount of hydrophobicity and hydrophilicity. Factors such as molecular weight and chain length of polymer, size, lipid concentration, and polymer concentrations may play a very important role in determination of the optimal micellar formulation. However, so far there have been no phospholipid micelles studies performed that evaluate the parameters for optimal formulation and activity.
Conversely, many studies of block copolymer, amphiphilic polymers, micelles have been done. Nivaggioli et al., Langmuir. 11 (3):730-737 (1995) tested block copolymer micelles of different pluronic copolymers (PEO-PPO-PEO) at a constant temperature and concentrations. The authors found that the increase in the molecular weight of the copolymer leads to an increase in the hydrodynamic size, thus suggesting an increase in the hydrophobic core size. Thus, the increase in micelle size due to the molecular weight and chain length would lead to an increase in uptake by RES. Therefore, high molecular weight and chain length decreases circulation time and hence the half-life of the SSM. Overall, the authors found PEO to be the most promising copolymer for SSM stability. Moreover, Carey and co-workers have determine that significant increase in the polymer concentration above the CMC leads to the formation of rod-like micelles causing an increase in the viscosity of the solution (Carey et al., Arch. Inter Med. 130:506-527 (1972); Almgren et al., Colloid Polym. Sci. 273:2-15 (1995)). Therefore, the elongated micelles increase the hydrophobicity of the micelles and may allow more of the non-polar drug to be encapsulated.
From these block co-polymers, amphiphilic compounds, one can infer that the study of parameters that optimize the formulation and activity of phospholipid micelle stability to be very relevant, and should be considered in the future.
The utilization of SSM as drug delivery system is a fairly new application, especially as therapeutic and diagnostic agents. As Trubetskoy et al., Proceed. Intern. Symp. Control. Tel. Bioact. Mater. 22:452453 (1995) pointed out, almost every possible drug administration route has benefited from the use of micellar drug formation in terms of increased bioavailability or reduced adverse effects. The small size of the micellar formulation allows for their penetration of blood brain barrier making it an ideal carrier for treatment of CNS diseases, such as Alzheimer's disease. Recently, SSM have been used as diagnostic agents using MRI and STM techniques (Trubetskoy et al., Proceed. Intern. Symp. Control. Test. Bioact. Mater., 22:452-453 (1995); Zamie et al., Collids and Suraces A: Physiocochemical and Engineering Aspects. 112:19-24 (1996)). In both cases SSM were incorporation with either a dye or paramagnetic agents followed by parenteral administration and visualization. In both cases the half-life of the SSM was at least 2 hours.
Also of interest to the present invention is the disclosure of Friedman et al., U.S. Pat. No. 5,514,670 which relates to submicron emulsions for delivery of bioactive peptides including vasoactive intestinal peptide analog. The submicron particles are said to have a weighted average diameter of 10 to 600 nm, more preferably 30 to 500 nm and most preferably 70 to 300 nm.
Of further interest to the present invention is calmodulin (CaM) which is an ubiquitous 17 kd protein that is found widely in the body and has many functions. Calmodulin functions mainly as a regulatory protein and serves as a sensor for calcium ions. The binding of Ca 2+ to four sites in calmodulin induces the formation of a-helix and other conformational changes that convert it from an inactive to an active form. The activated calmodulin in turn binds to many enzymes and proteins in the cell and modifies their activity. The globular structure of CaM hides hydrophobic binding sites for proteins that are exposed upon CaM interactions with Ca.sup.2+ ions and/or membrane phospholipids (Chiba et al., Life Sciences 47:953-960 (1990); Damrongehai et al., Bioconjugate Chem., 6:264-268 (1995)). Bolin, Neurochem. Int. 23:197-214 (1993) found that VIP is a potent stimulant of Ca.sup.2+ binding to calmodulin suggesting a correlation of VIP interactions with CaM and specific cellular regulatory activities.
Paul et al., Neurochem. Int. 23:197-214 (1993) also reported that internalized VIP had the ability to directly bind to calmodulin (CaM), and that it inhibited both phosphodiesterase as well as the calmodulin-dependent myosin light chain kinase activity. This observation supports a functional role for VIP-CaM complex (Stallwood et al., J. Biol. Chem. 267:19617-19621 (1992); Shiaga et al., Biochem. J. 300:901-905 (1994), therefore suggesting that calmodulin, a multifunctional protein responsible for the regulation of many different signaling enzymes, could be an intracellular receptor for VIP (Paul et al., Neurochem. Int. 23:197-214 (1993). Thus, VIP may regulate signal transduction by CaM association. Moreover, CaM is also found in extracellular fluid and cerebrospinal fluid and that it is actively secreted by cells (Paul et al., Neurochem. Int. 23:197-214 (1993)), thus the VIP-CAM complex may protect the peptide from protease digestion. Ca.sup.2+ ions and lipids are known to effect the peptide-CaM interactions. VIP and Ca.sup.2+ binding by CaM is cooperative, in that calcium ion (Ca.sup.2+) binding to receptors facilitates VIP binding to CaM and vise versa. Phospholipase treatment has been shown to inhibit VIP binding in intact membranes and modulates the binding by solubilizing VIP-binding protein fractions (Paul et al., Ann. N.Y. Acad. Sci. 527:282-295 (1988)). Thus, the biochemical consequences of VIP-CaM binding depends on the identity of CaM binding site, and conformational changes induced by VIP-CaM binding.
Thus, there exists a need in the art to provide further improvements in the use of micelle technology for the therapeutic and diagnostic administration of bioactive molecules. More specifically, there remains a desire in the art for improved methods for administration of amphipathic peptides including, but not limited to, members of the VIP/GRF family of peptides associated with phospholipids in order to achieve a more prolonged and effective therapeutic effect.